THURSDAY, Sept. 29, 2005 (HealthDay News) -- Researchers are reporting what they describe as the first clear picture of how and why consciousness fades as someone falls asleep.
A technique called transcranial magnetic stimulation shows that "the brain breaks down into little islands that can't talk to one another," explained Dr. Guilio Tononi, a professor of psychiatry at the University of Wisconsin-Madison, and lead author of a report on the work in the Sept. 30 issue of Science.
Tononi and his colleagues placed paddle-like devices that generate a magnetic field over the heads of volunteers, producing electrical pulses that penetrated the skull to stimulate brain cell activity.
Targeted cells in the cerebral cortex, which controls perception and thought, responded by sending signals to other cells in the cortex. Those signals travel freely when a person is awake, but no such signals are sent when someone is in deep sleep, the researchers reported.
The researchers said the study helps clarify thought about consciousness, a subject that is poorly understood by scientists. Tononi has theorized that conscious thought depends on the ability of the brain to integrate information. The findings help verify that belief, he said.
The brain is active during sleep, Tononi said; the vivid dreams that occur late at night testify to that. But the research subjects report no such conscious experiences early in the night, he said. The study indicates that disconnection of brain centers after sleep accounts for the difference.
When people are awake, the electrical pulses produced signals that traveled great distances in the brain, Tononi explained. "During sleep early in the night the response is short-lived and doesn't propagate at all. Somehow, it doesn't travel anywhere."
In addition to helping solve the mystery of consciousness, the new study may help in the development of diagnostic and treatment methods for neurological and psychiatric disorders, the researchers wrote.
The study got a mixed response from Robert Stickgold, an associate professor of psychiatry at Harvard Medical School and Beth Israel Deaconess Medical Center, whose work is cited in the Wisconsin report.
"He has plainly and elegantly demonstrated a breakdown in the ability of cortical areas to interact normally as we fall asleep," Stickgold said. "But he hasn't provided any reason to think that this is related to the changes in consciousness as we fall asleep."
Many changes occur in the brain as we fall asleep, Stickgold said. "Neurochemicals are released in the brain, EEG [electroencephalograph] patterns change, the gating of sensory stimuli changes. The changes that he has so nicely documented may or may not play a role in the change of consciousness."
There's a basic problem in the field, he said: "Scientists have nothing approaching an understanding of why we are conscious when we are awake or, indeed, why we are awake. So looking for what changes cause a loss of consciousness is a very difficult question because we don't know what we're looking for."
"I don't think this adds anything substantive about consciousness. It does add some information about the changes in brain function that accompany the shift to sleep, in a very elegant and beautiful way to show it," he said.
Dr. Marcello Massimini, a research associate at the Wisconsin university's Psychiatric Institute and Clinics and a member of the research team, disagreed.
"What is important is that this observation is the first experimental confirmation of a precise prediction from a theory formulated in the last 10 or 15 years by Dr. Tononi," he said. "The prediction was that it was not the level of activity of the brain but the ability of distant parts of the brain to communicate with each other that constitutes consciousness."
You can learn about sleep and how it is studied from Washington University.